1. Field of the Invention
This invention relates to methods and apparatus for controlling the current in an inductive device from a source of direct current electric power with high energy efficiency, even though the supply voltage from the source may be subject to considerable variations.
An application of the direct current control to operate a solenoid employed in well logging apparatus is disclosed in application Ser. No. 169,763 filed on July 17, 1980, now U.S. Pat. No. 4,336,564 issued on June 22, 1982.
2. The Prior Art
The presently existing techniques for controlling the DC current in an inductive device may be placed in three major classes as follows:
(a) Varying the Effective Voltage at the Load
The technique makes use of the relationship that the current, through a fixed resistance, is directly proportional to the voltage applied. The voltage applied to the load is monitored and the power source is directly or indirectly adjusted to maintain the required voltage. Direct control of the power source (e.g. adjusting the current in the field winding of a generator) is most often impractical or impossible. Indirect control is accomplished by placing some form of electronic voltage regulator between the power source and the load. A series type regulator allows for rapid and precise control of the applied voltage but often dissipates 60%-90% of the supplied power. A switching type regulator normally dissipates less than 20% of the supplied power but suffers from slow response to step changes in output voltage requirements. The power inductors and the filter capacitors are often exotic and bulky for high current applications, although this is offset by the reduced needs of heat sinks.
A limitation of using voltage control to set the current in the load is if the load resistance changes (within one unit or from unit to unit), the current will change in inverse proportion since the feedback circuits that are usually employed have no means of detecting this change of resistance.
(b) Passively Limiting the Current Through the Load
This technique makes use of the relationship that current is inversely proportional to resistance with a constant applied voltage. Although the least energy efficient of the three techniques, this approach is most commonly used due to its simplicity. A resistor is installed between the load and the power source. The value of the resistor is chosen such that the combined series resistance of the control resistor and the load limit the current to the desired value. If different values of currents are needed in one application, different resistor values are installed in the circuit via mechanical or electronic switches. This approach suffers several serious setbacks. Load resistance changes will change the load current, although not to the same extent as with voltage control. The resistor will dissipate as much or more power than a series regulator does (it is in fact a very simple series regulator). The most important limitation occurs when the supply voltage varies.
Since the resistance value of the load and control resistor combination remains constant, the supplied load current will vary proportionally with the supply voltage. This lack of regulation can be intolerable in most situations. This approach is also lacking in energy and volume efficiencies. The resistor value is designed to produce the desired current at the minimum supplied voltage. As the voltage increases, the supplied current increases proportionately. The power dissipation, however, increases according to the square of the current change (e.g., a doubling of the input voltage doubles the current, but the power dissipation increase by four times). The resistor, then must be of a power rating to withstand the stresses at the maximum voltage.
(c) Actively Limiting the Current Through the Load
In this technique, an active device (e.g., a transistor) is used to limit the current supplied to the load. A current sensing element (e.g., a resistor) is placed in series with the load and the voltage across this element is monitored. The control device is then set via electronics to adjust its effective resistance to limit the current to the desired amount. This system has merits in that the supplied current remains constant whether the load resistance changes or the input voltage varies. The power dissipation is similar to that of a series pass regulator. If the current range required is large, the sensing element may present a problem. A value that develops sufficient feedback voltage at low currents may be too large to allow the high end of the current range to be used at minimum supply voltage (too much resistance in the line).